If you're required to file a federal return for 2020, you can use Form 1040 (U.S. Individual Income Tax Return) if any of these are true:
- You're a U.S. citizen;
- You're a resident of the U.S. or Puerto Rico;
- You have a Permanent Resident Card ("green card");
- You're a nonresident alien married to a U.S. citizen/resident alien with whom you will file a joint return; or
- You meet the substantial presence test, which means you were physically present in the U.S. for at least 31 days in 2020 and at least 183 days in 2018 through 2020 according to this formula:
2020 days + (2019 days)/3 + (2018 days)/6 >= 183
Example: Joachim works 3 months (90 days) every summer as a camp counselor in the U.S. He meets the first part of the test (at least 31 days in 2020), but fails the 183-day test:
31 + 90/3 + 90/6 = 31+30+15 = 76 days
Because he didn't meet the substantial presence test, Joachim is considered a nonresident alien for federal tax purposes and cannot file a 1040. Instead, he must file a 1040-NR.
In addition to those who don't pass the substantial presence test, nonresident aliens also include:
- Foreign nationals
- Anybody who is not a U.S. citizen, U.S. or Puerto Rico resident alien, or doesn't hold a green card
- A person who has income from or participates in a U.S.-sourced trade or business
Unlike the 1040, there's no minimum income threshold for filing a 1040-NR. In other words, nonresident aliens with U.S. income, regardless of amount, are required to file a 1040-NR.